Aluminium alloy containing magnesium and silicon

ABSTRACT

An aluminum alloy and a process for treating the aluminum alloy. The alloy contains 0.5 to 2.5% by weight of an alloying mixture of magnesium and silicon, in which the molar ratio of Mg/Si is 0.70 to 1.25, the alloy optionally containing an additional amount of silicon up to about ⅓ of any iron, manganese and chromium in the alloy, as expressed by weight percent, the balance of the alloy being aluminum, optional alloying elements and unavoidable impurities. The process entails an ageing technique that includes a first stage in which an extrusion of the aluminum alloy is heated at a rate above 100° C./hour to 100-170° C., a second stage in which the extrusion is heated at a rate of 5 to 50° C./hour to a final hold temperature, wherein the total ageing operation is performed in 3 to 24 hours.

This application claims benefit of International Application No.PCT/EP99/00939, filed Feb. 12, 1999.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The invention relates to a process of treating an aluminum alloyconsisting of

0,5-2,5% by weight of an alloying mixture of magnesium and silicon, themolar ration of Mg/Si lying between 0,70 and 1,25,

an additional amount of Si equal to ⅓ of the amount of Fe, Mn and Cr inthe alloy, as expressed by % by weight,

other alloying elements and unavoidable impurities, and

the rest being made up of aluminium,

which alloy after cooling has been submitted to homogenising, preheatingbefore extrusion and ageing, which ageing takes place after extrusion asa dual step ageing operation to a final hold temperature between 160° C.and 220° C.

(2) Description of the Related Art

A process for ageing aluminum alloys containing magnesium and silicon(Al—Mg—Si) is described in WO 95.06769. According to this publicationthe ageing is performed at a temperature between 150 and 200° C., andthe rate of heating is between 10-100° C./hour preferably 10-70°C./hour. As an alternative to this, a two-step heating schedule isproposed, wherein a hold temperature in the range of 80-140° C. issuggested in order to obtain an overall heating rate within the abovespecified range.

It is generally known that higher total amounts of Mg and Si will have apositive effect on the mechanical properties of the final product,whereas this has a negative effect on the extrudability of the aluminiumalloy. It has previously been anticipated that the hardening phase inthe Al—Mg—Si alloys had a composition close to Mg₂Si. However, it wasalso known that an excess of Si produced higher mechanical properties.

Later experiments have shown that the precipitation sequence is quitecomplex and that except for the equilibrium phase, the phases involveddo not have the stoichiometric ratio Mg₂Si. In a publication of S. J.Andersen, et. al, Acta mater, Vol. 46 No. 9 p. 3283-3298 of 1998 it hasbeen suggested that one of the hardening phases in Al—Mg—Si alloys has acomposition close to Mg₅Si₆.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an aluminum alloy and a process fortreating the aluminum alloy which results in the alloy having bettermechanical properties and better extrudability as compared totraditional aluminium alloys. In particular, the alloy contains 0.5 to2.5% by weight of an alloying mixture of magnesium and silicon, in whichthe molar ratio of Mg/Si is 0.70 to 1.25, the alloy optionallycontaining an additional amount of silicon up to about ⅓ of any iron,manganese and chromium in the alloy, as expressed by weight percent, thebalance of the alloy being aluminum, optional alloying elements andunavoidable impurities. The process for treating this alloy entails anageing technique that includes a first stage in which an extrusion ofthe aluminum alloy is heated with a heating rate above 100° C./hour to atemperature between 100-170° C., a second stage in which the extrusionis heated with a heating rate between 5 and 50° C./hour to a final holdtemperature, and in that the total ageing cycle is performed in a timeof 3 to 24 hours.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing five different ageing cycles evaluated withdifferent Al—Mg—Si alloys of this invention.

DETAILED DESCRIPTION OF THE INVENTION

The optimum Mg/Si ratio is the one where all the available Mg and Si istransformed into Mg₅Si₆ phases. This combination of Mg and Si gives thehighest mechanical strength with the minimum use of the alloyingelements Mg and Si. It has been found that the maximum extrusion speedis almost independent of the Mg/Si ratio. Therefore, with the optimumMg/Si ratio the sum of Mg and Si is minimised for a certain strengthrequirement, and this alloy will thus also provide the bestextrudability. By using the composition according to the inventioncombined with the dual rate ageing procedure according to the invention,it has been obtained that the strength and extrudability are maximisedwith a minimum total ageing time.

In addition to the Mg₅Si₆ phase there is also another hardening phasewhich contains more Mg than the Mg₅Si₆ phase. However, this phase is notas effective, and does not contribute so much to the mechanical strengthas the Mg₅Si₆ phase. On the Si rich side of the Mg₅Si₆ phase there ismost probably no hardening phase, and lower Mg/Si ratios than ⅚ will notbe beneficial.

The positive effect on the mechanical strength of the dual rate ageingprocedure can be explained by the fact that a prolonged time at lowtemperature generally enhances the formation of a higher density ofprecipitates of Mg—Si. If the entire ageing operation is performed atsuch temperature, the total ageing time will be beyond practical limitsand the throughput in the ageing ovens will be too low. By a slowincrease of the temperature to the final ageing temperature, the highnumber of precipitates nucleated at the low temperature will continue togrow. The result will be a high number of precipitates and mechanicalstrength values associated with low temperature ageing but with aconsiderably shorter total ageing time.

A two step ageing also give improvements in the mechanical strength, butwith a fast heating from the first hold temperature to the second holdtemperature there is substantial chance of reversion of the smallestprecipitates, with a lower number of hardening precipitates and thus alower mechanical strength as a result. Another benefit of the dual rateageing procedure as compared to normal ageing and also two step ageing,is that a slow heating rate will ensure a better temperaturedistribution in the load. The temperature history of the extrusions inthe load will be almost independent of the size of the load, the packingdensity and the wall thickness' of the extrusions. The result will bemore consistent mechanical properties than with other types of ageingprocedures.

As compared to the ageing procedure described in WO 95.06759 where theslow heating rate is started from the room temperature, the dual rateageing procedure will reduce the total ageing time by applying a fastheating rate from room temperature to temperatures between 100 and 170°C. The resulting strength will be almost equally good when the slowheating is started at an intermediate temperature as if the slow heatingis started at room temperature.

Dependent upon the class of strength envisaged different compositionsare possible within the general scope of the invention.

So it is possible to have an aluminium alloy with a tensile strength inthe class F19-F22, the amount of alloying mixture of magnesium ofsilicon being between 0,60 and 1,10% by weight. For an alloy with atensile strength in the class F25-F27, it is possible to use analuminium alloy containing between 0,80 and 1,40 by weight of analloying mixture of magnesium and silicon and for an alloy with atensile strength in the class F29-F31, it is possible to use analuminium alloy containing between 1,10 and 1,80% by weight of thealloying mixture of magnesium and silicon.

Preferably and according to the invention a tensile strength in theclass F19 (185-220 MPa) is obtained by an alloy containing between 0,60and 0,80% by weight of the alloying mixture, a tensile strength in theclass F22 (215-250 MPa) by an alloy containing between 0,70 and 0,90% byweight of the alloying mixture, a tensile strength in the class F25(245-270 MPa) by an alloy containing between 0,85 and 1,15% by weight ofthe alloying mixture, a tensile strength in the class F27 (265-290 MPa)by an alloy containing between 0,95 and 1,25% by weight of the alloyingmixture, a tensile strength in the class F29 (285-310 MPa) by an alloycontaining between 1,10 and 1,40% by weight of the alloying mixture, anda tensile strength in the class F31 (305-330 MPa) by an alloy containingbetween 1,20 and 1,55% by weight of the alloying mixture.

With additions of Cu, which as a rule of thumb increases the mechanicalstrength by 10 MPa per 0.10 wt. % Cu, the total amount of Mg and Si canbe reduced and still match a strength class higher than the Mg and Siadditions alone would give.

For the reason described above it is preferred that the molar ratioMg/Si lies between 0.75 and 1.25 and more preferably between 0.8 and1.0.

In a preferred embodiment of the invention the final ageing temperatureis at least 165° C. and more preferably the ageing temperature is atmost 205° C. When using these preferred temperatures it has been foundthat the mechanical strength is maximised while the total ageing timeremains within reasonable limits.

In order to reduce the total ageing time in the dual rate ageingoperation it is preferred to perform the first heating stage at thehighest possible heating rate available, while as a rule is dependentupon the equipment available. Therefore, it is preferred to use in thefirst heating stage a heating rate of at least 100° C./hour.

In the second heating stage the heating rate must be optimised in viewof the total efficiency in time and the ultimate quality of the alloy.For that reason the second heating rate is preferably at least 7°C./hour and at most 30° C./hour. At lower heating rates than 7° C./hourthe total ageing time will be long with a low throughput in the ageingovens as a result, and at higher heating rates than 30° C./hour themechanical properties will be lower than ideal.

Preferably, the first heating stage will end up at 130-160° C. and atthese temperatures there is a sufficient precipitation of the Mg₅Si₆phase to obtain a high mechanical strength of the alloy. A lower endtemperature of the first stage will generally lead to an increased totalageing time. Preferably the total ageing time is at most 12 hours.

In order to have an extruded product with almost all the Mg and Si insolid solution before the ageing operation, it is important to controlthe parameters during extrusion and cooling after extrusion. With theright parameters this can be obtained by normal preheating. However, byusing a so-called overheating process described in EP 0302623, which isa preheating operation where the alloy is heated to a temperaturebetween 510 and 560° C. during the preheating operation beforeextrusion, after which the billets are cooled to normal extrusiontemperatures, this will ensure that all the Mg and Si added to the alloyare dissolved. By proper cooling of the extruded product the Mg and Siare maintained solved and available for forming hardening precipitatesduring the ageing operation.

For low alloy compositions the solutionising of Mg and Si can beobtained during the extrusion operation without overheating if theextrusion parameters are correct. However, with higher alloycompositions normal preheating conditions are not always enough to getall Mg and Si into solid solution. In such cases overheating will makethe extrusion process more robust and always ensure that the all the Mgand Si are in solid solution when the profile comes out of the press.

Other characteristics and advantages will be clear from the followingdescription of a number of tests done with alloys according to theinvention.

EXAMPLE 1

Eight different alloys with the composition given in Table 1 were castas Ø95 mm billets with standard casting conditions for 6060 alloys. Thebillets were homogenised with a heating rate of approximately 250°C./hour, the holding period was 2 hours and 15 minutes at 575° C., andthe cooling rate after homogenisation was approximately 350° C./hour.The logs were finally cut into 200 mm long billets.

TABLE 1 Alloy Si Mg Fe Total Si + Mg 1 0,34 0,40 0,20 0,74 2 0,37 0,360,19 0,73 3 0,43 0,31 0,19 0,74 4 0,48 0,25 0,20 0,73 5 0,37 0,50 0,180,87 6 0,41 0,47 0,19 0,88 7 0,47 0,41 0,20 0,88 8 0,51 0,36 0,19 0,87

The extrusion trial was performed in an 800 ton press equipped with aØ100 mm container, and an induction furnace to heat the billets beforeextrusion.

The die used for the extrudability experiments produced a cylindricalrod with a diameter of 7 mm with two ribs of 0.5 mm width and 1 mmheight, located 180° apart.

In order to get good measurements of the mechanical properties of theprofiles, a separate trial was run with a die which gave a 2*25 mm² bar.The billets were preheated to approximately 500° C. before extrusion.After extrusion the profiles were cooled in still air giving a coolingtime of approximately 2 min down to temperatures below 250° C. Afterextrusion the profiles were stretched 0.5%. The storage time at roomtemperature were controlled before ageing. Mechanical properties wereobtained by means of tensile testing.

The complete results of the extrudability tests for these alloys areshown in table 2 and 3.

TABLE 2 Extrusion tests for alloys 1-4 Ram Speed Billet TemperatureAlloy no. mm/sec. ° C. Remarks 1 16 502 OK 1 17 503 OK 1 18 502 Tearing1 17 499 OK 1 19 475 OK 1 20 473 OK 1 21 470 Tearing 2 16 504 OK 2 17503 Small Tearing 2 18 500 Tearing 2 20 474 OK 2 19 473 OK 2 18 470 OK 221 469 Small Tearing 3 17 503 Tearing 3 16 505 OK 3 15 504 OK 3 19 477OK 3 18 477 OK 3 20 472 OK 3 21 470 Tearing 4 17 504 OK 4 18 505 Tearing4 16 502 OK 4 19 477 OK 4 20 478 OK 4 20 480 Small Tearing 4 21 474Tearing

For alloys 1-4 which have approximately the same sum of Mg and Si butdifferent Mg/Si maximum extrusion speed before tearing is approximatelythe same at billet temperatures.

TABLE 3 Extrusion tests for alloys 5-8 Ram Speed Billet TemperatureAlloy no. mm/sec. ° C. Remarks 5 14 495 OK 5 14,5 500 Tearing 5 15 500Tearing 5 14 500 Small Tearing 5 17 476 Tearing 5 16,5 475 OK 5 16,8 476Small Tearing 5 17 475 Tearing 6 14 501 Small Tearing 6 13,5 503 OK 6 14505 Tearing 6 14,5 500 Tearing 6 17 473 Tearing 6 16,8 473 Tearing 616,5 473 OK 6 16,3 473 OK 7 14 504 Tearing 7 13,5 506 Small Tearing 713,5 500 OK 7 13,8 503 Small Tearing 7 17 472 Small Tearing 7 16,8 476Tearing 7 16,6 473 OK 7 17 475 Tearing 8 13,5 505 OK 8 13,8 505 Tearing8 13,6 504 OK 8 14 505 Tearing 8 17 473 Small Tearing 8 17,2 474 SmallTearing 8 17,5 471 Tearing 8 16,8 473 OK

For alloys 5-8 which have approximately the same sum of Mg and Si butdifferent Mg/Si ratios, the maximum extrusion speed before tearing isapproximately the same at comparable billet temperatures. However, bycomparing alloys 1-4 which have a lower sum of Mg and Si with alloys5-8, the maximum extrusion speed is generally higher for alloys 1-4.

The mechanical properties of the different alloy aged at differentageing cycles are shown in tables 4-11.

As an explanation to these tables, reference is made to FIG. 1 in whichdifferent ageing cycles are shown graphically and identified by aletter. In FIG. 1 there is shown the total ageing time on the x-axis,and the temperature used is along the y-axis.

Furthermore the different columns have the following meaning:

Total time=Total ageing time for the ageing cycle.

Rm=ultimate tensile strength;

R_(PO2)=yield strength;

AB=elongation to fracture;

Au=uniform elongation.

All these data has been obtained by means of standard tensile testingand the numbers shown are the average of two parallel samples of theextruded profile.

TABLE 4 Alloy 1-0.40 Mg + 0.34 Si Total Time [hrs] Rm Rp02 AB Au A 3143,6 74,0 16,8 8,1 A 4 160,6 122,3 12,9 6,9 A 5 170,0 137,2 12,6 5,6 A6 178,1 144,5 12,3 5,6 A 7 180,3 150,3 12,3 5,2 B 3,5 166,8 125,6 12,96,6 B 4 173,9 135,7 11,9 6,1 B 4,5 181,1 146,7 12,0 5,4 B 5 188,3 160,812,2 5,1 B 6 196,0 170,3 11,9 4,7 C 4 156,9 113,8 12,6 7,5 C 5 171,9134,7 13,2 6,9 C 6 189,4 154,9 12,0 6,2 C 7 195,0 168,6 11,9 5,8 C 8199,2 172,4 12,3 5,4 D 7 185,1 140,8 12,9 6,4 D 8,5 196,5 159,0 13,0 6,2D 10 201,8 171,6 13,3 6,0 D 11,5 206,4 177,5 12,9 6,1 D 13 211,7 184,012,5 5,4 E 8 190,5 152,9 12,8 6,5 E 10 200,3 168,3 12,1 6,0 E 12 207,1176,7 12,3 6,0 E 14 211,2 185,3 12,4 5,9 E 16 213,9 188,8 12,3 6,6

TABLE 5 Alloy 2-0.36 Mg + 0.37 Si Total Time [hrs] Rm Rp02 AB Au A 3150,1 105,7 13,4 7,5 A 4 164,4 126,1 13,6 6,6 A 5 174,5 139,2 12,9 6,1 A6 183,1 154,4 12,4 4,9 A 7 185,4 157,8 12,0 5,4 B 3,5 175,0 135,0 12,36,3 B 4 181,7 146,6 12,1 6,0 B 4,5 190,7 158,9 11,7 5,5 B 5 195,5 169,912,5 5,2 B 6 202,0 175,7 12,3 5,4 C 4 161,3 114,1 14,0 7,2 C 5 185,7145,9 12,1 6,1 C 6 197,4 167,6 11,6 5,9 C 7 203,9 176,0 12,6 6,0 C 8205,3 178,9 12,0 5,5 D 7 195,1 151,2 12,6 6,6 D 8,5 208,9 180,4 12,5 5,9D 10 210,4 181,1 12,8 6,3 D 11,5 215,2 187,4 13,7 6,1 D 13 219,4 189,312,4 5,8 E 8 195,6 158,0 12,9 6,7 E 10 205,9 176,2 13,1 6,0 E 12 214,8185,3 12,1 5,8 E 14 216,9 192,5 12,3 5,4 E 16 221,5 196,9 12,1 5,4

TABLE 6 Alloy 3-0.31 Mg + 0.43 Si Total Time [hrs] Rm Rp02 AB Au A 3154,3 111,0 15,0 8,2 A 4 172,6 138,0 13,0 6,5 A 5 180,6 148,9 13,0 5,7 A6 189,7 160,0 12,2 5,5 A 7 192,5 164,7 12,6 5,3 B 3,5 187,4 148,9 12,36,3 B 4 193,0 160,3 11,5 5,9 B 4,5 197,7 168,3 11,6 5,1 B 5 203,2 177,112,4 5,5 B 6 205,1 180,6 11,7 5,4 C 4 170,1 127,4 14,3 7,5 C 5 193,3158,2 13,4 6,2 C 6 207,3 179,2 12,6 6,4 C 7 212,2 185,3 12,9 5,7 C 8212,0 188,7 12,3 5,6 D 7 205,6 157,5 13,2 6,7 D 8,5 218,7 190,4 12,7 6,0D 10 219,6 191,1 12,9 6,7 D 11,5 222,5 197,5 13,1 5,9 D 13 226,0 195,712,2 6,1 E 8 216,6 183,5 12,6 6,8 E 10 217,2 190,4 12,6 6,9 E 12 221,6193,9 12,4 6,6 E 14 225,7 200,6 12,4 6,0 E 16 224,4 197,8 12,1 5,9

TABLE 7 Alloy 4-0.25 Mg + 0.48 Si Total Time [hrs] Rm Rp02 AB Au A 3140,2 98,3 14,5 8,6 A 4 152,8 114,6 14,5 7,2 A 5 166,2 134,9 12,7 5,9 A6 173,5 141,7 12,8 5,7 A 7 178,1 147,6 12,3 5,2 B 3,5 165,1 123,5 13,36,4 B 4 172,2 136,4 11,8 5,7 B 4,5 180,7 150,2 12,1 5,2 B 5 187,2 159,512,0 5,6 B 6 192,8 164,6 12,1 5,0 C 4 153,9 108,6 13,6 7,7 C 5 177,2141,8 12,0 6,5 C 6 190,2 159,7 11,9 5,9 C 7 197,3 168,6 12,3 6,1 C 8197,9 170,6 12,5 5,6 D 7 189,5 145,6 12,3 6,4 D 8,5 202,2 171,6 12,6 6,1D 10 207,9 178,8 12,9 6,0 D 11,5 210,7 180,9 12,7 5,6 D 13 213,3 177,712,4 6,0 E 8 195,1 161,5 12,8 5,9 E 10 205,2 174,1 12,5 6,4 E 12 208,3177,3 12,8 5,6 E 14 211,6 185,9 12,5 6,3 E 16 217,6 190,0 12,4 6,2

TABLE 8 Alloy 5-0.50 Mg + 0.37 Si Total Time [hrs] Rm Rp02 AB Au A 3180,6 138,8 13,9 7,1 A 4 194,2 155,9 13,2 6,6 A 5 203,3 176,5 12,8 5,6 A6 210,0 183,6 12,2 5,7 A 7 211,7 185,9 12,1 5,8 B 3,5 202,4 161,7 12,86,6 B 4 204,2 170,4 12,5 6,1 B 4,5 217,4 186,7 12,1 5,6 B 5 218,9 191,512,1 5,5 B 6 222,4 198,2 12,3 6,0 C 4 188,6 136,4 15,1 10,0 C 5 206,2171,2 13,4 7,1 C 6 219,2 191,2 12,9 6,2 C 7 221,4 194,4 12,1 6,1 C 8224,4 202,8 11,8 6,0 D 7 213,2 161,5 14,0 7,5 D 8,5 221,5 186,1 12,6 6,7D 10 229,9 200,8 12,1 5,7 D 11,5 228,2 200,0 12,3 6,3 D 13 233,2 198,111,4 6,2 E 8 221,3 187,7 13,5 7,4 E 10 226,8 196,7 12,6 6,7 E 12 227,8195,9 12,8 6,6 E 14 230,6 200,5 12,2 5,6 E 16 235,7 207,9 11,7 6,4

TABLE 9 Alloy 6-0.47 Mg + 0.41 Si Total Time [hrs] Rm Rp02 AB Au A 3189,1 144,5 13,7 7,5 A 4 205,6 170,5 13,2 6,6 A 5 212,0 182,4 13,0 5,8 A6 216,0 187,0 12,3 5,6 A 7 216,4 188,8 11,9 5,5 B 3,5 208,2 172,3 12,86,7 B 4 213,0 175,5 12,1 6,3 B 4,5 219,6 190,5 12,0 6,0 B 5 225,5 199,411,9 5,6 B 6 225,8 202,2 11,9 5,8 C 4 195,3 148,7 14,1 8,1 C 5 214,1178,6 13,8 6,8 C 6 227,3 198,7 13,2 6,3 C 7 229,4 203,7 12,3 6,6 C 8228,2 200,7 12,1 6,1 D 7 222,9 185,0 12,6 7,8 D 8,5 230,7 194,0 13,0 6,8D 10 236,6 205,7 13,0 6,6 D 11,5 236,7 208,0 12,4 6,6 D 13 239,6 207,111,5 5,7 E 8 229,4 196,8 12,7 6,4 E 10 233,5 199,5 13,0 7,1 E 12 237,0206,9 12,3 6,7 E 14 236,0 206,5 12,0 6,2 E 16 240,3 214,4 12,4 6,8

TABLE 10 Alloy 7-0.41 Mg + 0.47 Si Total Time [hrs] Rm Rp02 AB Au A 3195,9 155,9 13,5 6,6 A 4 208,9 170,0 13,3 6,4 A 5 216,2 188,6 12,5 6,2 A6 220,4 195,1 12,5 5,5 A 7 222,0 196,1 11,5 5,4 B 3,5 216,0 179,5 12,26,4 B 4 219,1 184,4 12,2 6,1 B 4,5 228,0 200,0 11,9 5,8 B 5 230,2 205,911,4 6,1 B 6 231,1 211,1 11,8 5,5 C 4 205,5 157,7 15,0 7,8 C 5 225,2190,8 13,1 6,8 C 6 230,4 203,3 12,0 6,5 C 7 234,5 208,9 12,1 6,2 C 8235,4 213,4 11,8 5,9 D 7 231,1 190,6 13,6 7,6 D 8,5 240,3 208,7 11,4 6,3D 10 241,6 212,0 12,5 7,3 D 11,5 244,3 218,2 11,9 6,3 D 13 246,3 204,211,3 6,3 E 8 233,5 197,2 12,9 7,6 E 10 241,1 205,8 12,8 7,2 E 12 244,6214,7 11,9 6,5 E 14 246,7 220,2 11,8 6,3 E 16 247,5 221,6 11,2 5,8

TABLE 11 Alloy 8-0.36 Mg + 0.51 Si Total Time [hrs] Rm Rp02 AB Au A 3200,1 161,8 13,0 7,0 A 4 212,5 178,5 12,6 6,2 A 5 221,9 195,6 12,6 5,7 A6 222,5 195,7 12,0 6,0 A 7 224,6 196,0 12,4 5,9 B 3,5 222,2 186,9 12,66,6 B 4 224,5 188,8 12,1 6,1 B 4,5 230,9 203,4 12,2 6,6 B 5 231,1 211,711,9 6,6 B 6 232,3 208,8 11,4 5,6 C 4 215,3 168,5 14,5 8,3 C 5 228,9194,9 13,6 7,5 C 6 234,1 206,4 12,6 7,1 C 7 239,4 213,3 11,9 6,4 C 8239,1 212,5 11,9 5,9 D 7 236,7 195,9 13,1 7,9 D 8,5 244,4 209,6 12,2 7,0D 10 247,1 220,4 11,8 6,7 D 11,5 246,8 217,8 12,1 7,2 D 13 249,4 223,711,4 6,6 E 8 243,0 207,7 12,8 7,6 E 10 244,8 215,3 12,4 7,4 E 12 247,6219,6 12,0 6,9 E 14 249,3 222,5 12,5 7,1 E 16 250,1 220,8 11,5 7,0

Based upon these results the following comments apply.

The ultimate tensile strength (UTS) of alloy no. 1 is slightly below 180MPa after ageing with the A—cycle and 6 hours total time. With the dualrate ageing cycles the UTS values are higher, but still not more than190 MPA after a 5 hours B—cycle, and 195 MPa after a 7 hours C—cycle.With the D—cycle the UTS values reaches 210 MPa but not before a totalageing time of 13 hours.

The ultimate tensile strength (UTS) of alloy no. 2 is slightly above 180MPa after the A—cycle and 6 hours total time. The UTS values are 195 MPaafter a 5 hours B—cycle, and 205 MPa after a 7 hours C—cycle. With theD—cycle the UTS values reaches approximately 210 MPa after 9 hours and215 MPa after 12 hours.

Alloy no. 3 which is closest to the Mg5Si₆ line on the Mg rich side,shows the highest mechanical properties of alloys 1-4. After the A—cyclethe UTS is 190 MPa after 6 hours total time. With a 5 hours B—cycle theUTS is close to 205 MPa, and slightly above 210 MPa after a 7 hoursC—cycle. With the D—ageing cycle of 9 hours the UTS is close to 220 MPa.

Alloy no. 4 shows lower mechanical properties than alloys 2 and 3. Afterthe A—cycle with 6 hours total time the UTS is not more than 175 MPa.With the D—ageing cycle of 10 hours the UTS is close to 210 MPa.

These results clearly demonstrate that the optimum composition forobtaining the best mechanical properties with the lowest sum of Mg andSi, is close to the Mg₅Si₆ line on the Mg rich side.

Another important aspect with the Mg/Si ratio is that a low ratio seemto give shorter ageing times to obtain the maximum strength.

Alloys 5-8 have a constant sum of Mg and Si that is higher than foralloys 1-4. As compared to the Mg₅Si₆ line, all alloys 5-8 are locatedon the Mg rich side of Mg₅Si₆.

Alloy no. 5 which is farthest away from the Mg₅Si₆ line shows the lowestmechanical properties of four different alloys 5-8. With the A—cyclealloy no. 5 has a UTS value of approximately 210 MPa after 6 hours totaltime. Alloy no. 8 has an UTS value of 220 MPa after the same cycle. Withthe C—cycle of 7 hours total time the UTS values for alloys 5 and 8 are220 and 240 MPa, respectively. With the D—cycle of 9 hours the UTSvalues are approximately 225 and 245 MPa.

Again, this shows that the highest mechanical properties are obtainedwith alloys closest to the Mg₅Si₆ line. As for alloys 1-4, the benefitsof the dual rate ageing cycles seem to be highest for alloys closest tothe Mg₅Si₆ line.

The ageing times to maximum strength seem to be shorter for alloys 5-8than for alloys 1-4. This is as expected because the ageing times arereduced with increased alloy content. Also, for alloys 5-8 the ageingtimes seem to be somewhat shorter for alloy 8 than for alloy 5.

The total elongation values seem to be almost independent of the ageingcycle. At peak strength the total elongation values, AB, are around 12%,even though the strength values are higher for the dual rate ageingcycles.

EXAMPLE 2

Example 2 shows the ultimate tensile strength of profiles from directlyand overheated billets of a 6061 alloy. The directly heated billets wereheated to the temperature shown in the table and extruded at extrusionspeeds below the maximum speed before deterioration of the profilesurface. The overheated billets were preheated in a gas fired furnace toa temperature above the solvus temperature for the alloy and then cooleddown to a normal extrusion temperature shown in table 12. Afterextrusion the profiles were water cooled and aged by a standard ageingcycle to peak strength.

TABLE 12 Ultimate tensile strength (UTS) in different positions ofprofiles from directly heated and overheated billets of a AA6061 alloy.Billet temperature UTS (front) UTS (middle) UTS (rear) Preheating ° C.MPa MPa MPa Dir. Heated 470 287.7 292.6 293.3 Dir. Heated 472 295.3293.9 296.0 Dir. Heated 471 300.8 309.1 301.5 Dir. Heated 470 310.5318.1 315.3 Dir. Heated 482 324.3 312.6 313.3 Dir. Heated 476 327.1334.0 331.9 Dir. Heated 476 325.7 325.0 319.5 Dir. Heated 475 320.2319.0 318.8 Dir. Heated 476 316.0 306.4 316.0 Dir. Heated 485 329.1329.8 317.4 Dir. Heated 501 334.7 324.3 331.2 Dir. Heated 499 332.6327.8 322.9 Dir. Heated 500 327.8 329.8 318.8 Dir. Heated 505 322.9322.2 318.1 Dir. Heated 502 325.7 329.1 334.7 Dir. Heated 506 336.0323.6 311.2 Dir. Heated 500 329.1 293.9 345.0 Dir. Heated 502 331.2332.6 335.3 Dir. Heated 496 318.8 347.8 294.6 Average UTS and standard320.8/13.1 319.6/14.5 317.6/13.9 deviation for directly heated billetsOverheated 506 333.3 325.7 331.3 Overheated 495 334.0 331.9 335.3Overheated 493 343.6 345.0 333.3 Overheated 495 343.6 338.8 333.3Overheated 490 339.5 332.6 327.1 Overheated 499 346.4 332.6 331.2Overheated 496 332.6 335.3 331.9 Overheated 495 330.5 331.2 322.9Overheated 493 332.6 334.7 333.3 Overheated 494 331.2 334.0 328.4Overheated 494 329.1 338.8 337.4 Overheated 459 345.7 337.4 344.3Overheated 467 340.2 338.1 330.5 Overheated 462 344.3 342.9 331.9Overheated 459 334.0 329.8 326.4 Overheated 461 331.9 326.4 324.3Average UTS and standard 337/5.9 334.7/5.2 331.4/5.0 deviation foroverheated billets

By utilising the overheating process the mechanical properties willgenerally be higher and also more consistent than without overheating.Also, with overheating the mechanical properties are practicallyindependent of the billet temperature prior to extrusion. This makes theextrusion process more robust with respect to providing high andconsistent mechanical properties, making it possible to operate at loweralloy compositions with lower safety margins down to the requirementsfor mechanical properties.

What is claimed is:
 1. A process of treating an aluminium alloycomprising 0.5 to 2.5 percent by weight of an alloying mixture ofmagnesium and silicon, the molar ratio of Mg/Si being 0.70 to 1.25,optionally an additional amount of Si equal to about ⅓ by weight percentof any amount of Fe, Mn and Cr in the alloy, unavoidable impurities, andbalance aluminum, the process comprising the steps of: casting, cooling,homogenising, preheating and extruding the alloy; and then ageing thealloy with a dual step ageing operation to a final hold temperature of160° C. to 220° C., the dual step ageing operation comprising a firststage in which the alloy is heated at a heating rate above 100° C./hourto a temperature of 100 to 170° C., and comprises a second stage inwhich the alloy is heated at a heating rate of 5 to 50° C./hour to thefinal hold temperature, the ageing operation being performed in a timeof 3 to 24 hours.
 2. A process according to claim 1, wherein the alloycontains 0.60 to 1.10 percent by weight of the alloying mixture ofmagnesium and silicon and has a tensile strength of 185 to 250 MPa afterthe ageing operation.
 3. A process according to claim 2, wherein thealloy contains 0.70 to 0.90 percent by weight of the alloying mixture ofmagnesium and silicon and has a tensile strength of 215 to 250 MPa afterthe ageing operation.
 4. A process according to claim 1, wherein thealloy contains 0.80 to 1.40 percent by weight of the alloying mixture ofmagnesium and silicon and has a tensile strength of 245 to 290 MPa afterthe ageing operation.
 5. A process according to claim 4, wherein thealloy contains 0.85 to 1.15 percent by weight of the alloying mixture ofmagnesium and silicon and has a tensile strength of 245 to 270 MPa afterthe ageing operation.
 6. A process according to claim 4, wherein thealloy contains 0.95 to 1.25 percent by weight of the alloying mixture ofmagnesium and silicon and has a tensile strength of 265 to 290 MPa afterthe ageing operation.
 7. A process according to claim 1, wherein thealloy contains 1.10 to 1.80 percent by weight of the alloying mixture ofmagnesium and silicon and has a tensile strength of 285 to 330 MPa afterthe ageing operation.
 8. A process according to claim 7, wherein thealloy contains 1.10 to 1.40 percent by weight of the alloying mixture ofmagnesium and silicon and has a tensile strength of 285 to 310 MPa afterthe ageing operation.
 9. A process according to claim 7, wherein thealloy contains 1.20 to 1.55 percent by weight of the alloying mixture ofmagnesium and silicon and has a tensile strength of 305 to 330 MPa afterthe ageing operation.
 10. A process according to claim 1, wherein thealloy contains 0.60 to 0.80 percent by weight of the alloying mixture ofmagnesium and silicon and has a tensile strength of 185 to 220 MPa afterthe ageing operation.
 11. A process according to claim 1, wherein themolar ratio of Mg/Si is at least 0.70.
 12. A process according to claim1, wherein the molar ratio of Mg/Si is at most 1.25.
 13. A processaccording to claim 1, wherein the final hold temperature is at least165° C.
 14. A process according to claim 1, wherein the final holdtemperature is at most 205° C.
 15. A process according to claim 1,wherein in the second stage the heating rate is at least 7° C./hour. 16.A process according to claim 1, wherein in the second stage the heatingrate is at most 30° C./hour.
 17. A process according to claim 1, whereinat the end of the first stage the temperature is between 130 and 160° C.18. A process according to claim 1, wherein the ageing operation isperformed in at least 5 hours.
 19. A process according to claim 1,wherein the ageing operation is performed in at most 12 hours.
 20. Aprocess according to claim 1, wherein during the preheating beforeextrusion the alloy is heated to a temperature between 510 and 550° C.,after which the alloy is cooled.